Re: [Patches] Add variant constexpr support for visit, comparisons and get

[Jonathan Wakely <jwakely at redhat dot com>]

On 30/11/16 19:29 -0800, Tim Shen wrote:
> On Wed, Nov 30, 2016 at 8:27 AM, Jonathan Wakely wrote:
> > On 26/11/16 21:38 -0800, Tim Shen wrote:
> > > +  template<typename _Type, bool = std::is_literal_type_v<_Type>>
> > >     struct _Uninitialized;
> >
> >
> > I'm still unsure that is_literal_type is the right trait here. If it's
> > definitely right then we should probably *not* deprecate it in C++17!
>
> No it's not right. We need this only because [basic.types]p10.5.3 (in n4606):
>
>  if it (a literal type) is a union, at least one of its non-static
> data members is of non-volatile literal type, ...
>
> is not implemented. In the current GCC implementation, however, all
> non-static data members need to be literal types, in order to create a
> literal union.
>
> With the current GCC implementation, to keep our goal, which is to
> make _Variadic_union literal type, we need to ensure that
> _Uninitialized<T> is literal type, by specializing on T:
> 1) If is_literal_type_v<T>, store a T;
> 2) otherwise, store a raw buffer of T.
>
> In the future, when [basic.types]p10.5.3 is implemented, we don't need
> is_literal_type_v.
>
> I'll add a comment here.

Thanks, that will stop me asking again and again in future ;-)

I think we want to get [basic.types] p10 implemented before we declare
C++17 support non-experimental, so we don't have to change
std::variant layout later.

> I didn't check for other compilers.

That's fine, the current approach should work for them too.